In computed topography (CT) imaging, portions of a patient are scanned and the density of materials contained therein are determined for various diagnostic and evaluation purposes. There is a continuous effort to increase CT imaging system scanning capabilities. Specifically, in CT imaging, it is desirable not only to be capable of determining density of scanned materials, but also to be able to distinguish between materials or combinations of materials that have similar densities.
For example, in certain testing procedures, in order to enhance visibility of blood and to better differentiate blood from other tissues or undesirable deposits within a vessel or organ, Iodide may be injected into the bloodstream of a patient. Combination of Iodide and water or blood, which consists mainly of water, and a combination of calcium deposits and soft tissue exhibit similar material densities, resulting in poor spatial and low contrast resolution between each combination and having effectively similar corresponding brightness levels when viewed by a practitioner. It is undesirable to have calcium build-up on inner linings of blood vessel walls. Thus, the practitioner, due to difficulty in discerning between the brightness levels of reconstructed CT images for the stated combinations, may not be able to determine whether there exists a calcium build-up in the blood vessels of the patient.
Referring now to FIG. 1, a cross-sectional view of a traditional CT tube assembly 10 is shown. CT imaging systems include a gantry that rotates at various speeds in order to create a 360° image. The gantry contains the CT tube assembly 10, which generates x-rays across a vacuum gap 12 between a single cathode 14 and an anode 16. In order to generate the x-rays, a large voltage potential is created across the vacuum gap 12 allowing electrons, in the form of an electron beam, to be emitted from the cathode 14 to a single target 18 of the anode 16. In releasing of the electrons, a filament contained within the cathode 14 is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target 18, whereby they are abruptly slowed down to emit x-rays and form an x-ray beam that passes through a CT tube window 20.
After passing through the CT tube window 20, the x-ray beam is filtered via a single filter 22. The filter 22 reduces the number of low energy x-rays that have energy levels below a predetermined energy level, thus reducing x-ray exposure to a patient. An example of a pre-patient energy spectrum plot of number of x-rays versus corresponding energy levels is shown in FIG. 2. A post-filter spectrum curve 24 overlays an approximate pre-filter spectrum curve 26. Notice that the spectrum curve 24 is single peaked and that the number of x-rays corresponding to energy levels below 40 KeV are significantly reduced, due to absorption by the filter 22.
The post filter x-rays pass through a portion of the patient and are detected by an x-ray detector array. As the x-rays pass through the patient, the x-rays become attenuated before impinging upon the detector array. X-ray attenuation measurements are generated by the x-ray detector corresponding to electrical signal response generated by the received x-rays having varying energy levels depending upon attenuation thereof. An x-ray image is reconstructed in response to the attenuation measurements.
The x-ray detector array generates an x-ray signal in response to the single peaked energy spectrum. Number of x-rays received by the detector is integrated over an average area of the detector and over a view time interval to generate an integrated signal. The integrated signal is directly related to densities of scanned materials of the patient. As is known in the art, it is difficult from the resulting energy spectrum and from inherent characteristics of integration to differentiate between similar material densities.
It would therefore be desirable to provide a CT system of energy discrimination to differentiate between different scanned materials and different scanned material combinations to increase CT scanning utility and capability. It would also be desirable for the CT system to be capable of performing energy discrimination with accuracy, clarity, and without increased x-ray exposure to a patient.